Is it permissible for a woman to put henna on her head while she is in a state of impurity, and should she wipe over the henna when performing ablution, or must she wash it?

Scholars differ on the ruling concerning wiping over henna placed on the head during Wudu' (ablution):

The Hanafis are of the view that it is permissible to wipe over it, explaining that water penetrates through the henna, or due to necessity. The majority of scholars (Malikis and Hanbalis) are of the view that it is necessary to remove the henna because it prevents water from reaching the head.

Some contemporary scholars, such as Sheikh Ibn Uthaymin and Sheikh Ibn Baz, have opted for the first opinion, which permits wiping over henna during Wudu'; this is because wiping is based on ease and leniency, unlike ghusl (ritual bath) from major impurity (janabah) or menstruation, for which it is necessary to remove it and ensure water reaches all hair roots. They held that wiping over henna does not invalidate purity.

If it is possible to remove the henna to avoid the difference of opinion, that would be better, unless the layer is thin enough for water to reach the hair, in which case there is no harm.